Method for Producing Water-Absorbing Polymer Particles by Suspension Polymerization

ABSTRACT

A process for producing water-absorbing polymer particles by suspension polymerization and thermal surface postcrosslinking, wherein the base polymer obtained by suspension polymerization has a centrifuge retention capacity of at least 37 g/g and the thermal surface postcrosslinking is conducted at 100 to 190° C.

The present invention relates to a process for producing water-absorbingpolymer particles by suspension polymerization and thermal surfacepostcrosslinking, wherein the base polymer obtained by suspensionpolymerization has a centrifuge retention capacity of at least 37 g/gand the thermal surface postcrosslinking is conducted at 100 to 190° C.

The production of water-absorbing polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, pages 69 to 117. The water-absorbingpolymer particles are typically produced by solution polymerization orsuspension polymerization.

Being products which absorb aqueous solutions, water-absorbing polymersare used to produce diapers, tampons, sanitary napkins and other hygienearticles, but also as water-retaining agents in market gardening.

The properties of the water-absorbing polymers can be adjusted via thelevel of crosslinking. With increasing level of crosslinking, there is arise in gel strength and a fall in absorption capacity.

To improve the use properties, for example permeability in the swollengel bed in the diaper and absorption under pressure, water-absorbingpolymer particles are generally postcrosslinked. This increases only thelevel of crosslinking of the particle surface, and in this way it ispossible to at least partly decouple absorption under pressure andcentrifuge retention capacity.

JP S63-218702 describes a continuous process for producingwater-absorbing polymer particles by suspension polymerization.

WO 2006/014031 A1describes a process for producing water-absorbingpolymer particles by suspension polymerization. At the high temperaturesin the thermal postcrosslinking, the fraction of hydrophobic solvent isdriven out.

WO 2008/068208 A1 likewise relates to a process for producingwater-absorbing polymer particles having a low proportion of hydrophobicsolvents by suspension polymerization.

It was an object of the present invention to provide an improved processfor producing water-absorbing polymer particles by suspensionpolymerization, wherein the water-absorbing polymer particles shouldhave a high centrifuge retention capacity (CRC), a high absorption undera pressure of 49.2 g/cm² (AUHL), a high sum total of centrifugeretention capacity (CRC) and absorption under a pressure of 49.2 g/cm²(AUHL), and a low level of extractables.

The object was achieved by a process for continuously producingwater-absorbing polymer particles by polymerizing a monomer solutioncomprising

-   -   a) at least one ethylenically unsaturated monomer which bears        acid groups and may have been at least partly neutralized,    -   b) optionally one or more crosslinkers,    -   c) at least one initiator,    -   d) optionally one or more ethylenically unsaturated monomers        copolymerizable with the monomers mentioned under a) and    -   e) optionally one or more water-soluble polymers,        the monomer solution being suspended in a hydrophobic organic        solvent during the polymerization, and thermally surface        postcrosslinking the resultant polymer particles by means of an        organic surface postcrosslinker, wherein the amount of        crosslinker b) is selected such that the polymer particles        before the surface postcrosslinking have a centrifuge retention        capacity of at least 37 g/g and the thermal surface        postcrosslinking is conducted at 100 to 190° C.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of waterand most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid and itaconicacid. Particularly preferred monomers are acrylic acid and methacrylicacid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids, such as styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a considerable influence on the polymerization. Theraw materials used should therefore have a maximum purity. It istherefore often advantageous to specially purify the monomers a).Suitable purification processes are described, for example, in WO2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitablemonomer a) is, for example, an acrylic acid purified according to WO2004/035514 A1 and comprising 99.8460% by weight of acrylic acid,0.0950% by weight of acetic acid, 0.0332% by weight of water, 0.0203% byweight of propionic acid, 0.0001% by weight of furfurals, 0.0001% byweight of maleic anhydride, 0.0003% by weight of diacrylic acid and0.0050% by weight of hydroquinone monomethyl ether.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The acid groups of the monomers a) may have been partly neutralized. Theneutralization is conducted at the monomer stage. This is typicallyaccomplished by mixing in the neutralizing agent as an aqueous solutionor preferably also as a solid. The degree of neutralization ispreferably from 25 to 95 mol %, more preferably from 30 to 80 mol % andmost preferably from 40 to 75 mol %, for which the customaryneutralizing agents can be used, preferably alkali metal hydroxides,alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof.

The monomers a) typically comprise polymerization inhibitors, preferablyhydroquinone monoethers, as storage stabilizers.

The monomer solution comprises preferably up to 250 ppm by weight,preferably at most 130 ppm by weight, more preferably at most 70 ppm byweight, and preferably at least 10 ppm by weight, more preferably atleast 30 ppm by weight and especially around 50 ppm by weight, ofhydroquinone monoether, based in each case on the unneutralized monomera). For example, the monomer solution can be prepared by using anethylenically unsaturated monomer bearing acid groups with anappropriate content of hydroquinone monoether.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized free-radically into thepolymer chain, and functional groups which can form covalent bonds withthe acid groups of the monomer a). In addition, polyvalent metal saltswhich can form coordinate bonds with at least two acid groups of themonomer a) are also suitable as crosslinkers b).

Crosslinkers b) are preferably compounds having at least twopolymerizable groups which can be polymerized free-radically into thepolymer network. Suitable crosslinkers b) are, for example,methylenebisacrylamide, ethylene glycol dimethacrylate, diethyleneglycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate,trimethylolpropane triacrylate, triallylamine, tetraallylammoniumchloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- andtriacrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632068 A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO2003/104301 A1 and DE 103 31 450 A1, mixed acrylates which, as well asacrylate groups, comprise further ethylenically unsaturated groups, asdescribed in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinkermixtures, as described, for example, in DE 195 43 368 A1, DE 196 46 484A1, WO 90/15830 A1 and WO 2002/032962 A2.

Preferred crosslinkers b) are pentaerythrityl triallyl ether,tetraallyloxyethane, methylenebismethacrylamide, 15-tuply ethoxylatedtrimethylolpropane triacrylate, polyethylene glycol diacrylate,trimethylolpropane triacrylate and triallylamine.

Very particularly preferred crosslinkers b) are methylenebisacrylamideand the polyethoxylated and/or -propoxylated glycerols which have beenesterified with acrylic acid or methacrylic acid to give di- ortriacrylates, as described, for example, in WO 2003/104301 A1.Methylenebisacrylamide and di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to methylenebisacrylamide, di- or triacrylates of 1-to 5-tuply ethoxylated and/or propoxylated glycerol. Most preferred aremethylenebisacrylamide and the triacrylates of 3- to 5-tuply ethoxylatedand/or propoxylated glycerol, especially methylenebisacrylamide and thetriacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker in the monomer solution is selected such thatthe water-absorbing polymer particles after the polymerization andbefore the thermal surface postcrosslinking (base polymer) have acentrifuge retention capacity (CRC) of at least 37 g/g, preferably atleast 38 g/g, more preferably at least 39 g/g, most preferably at least40 g/g. The centrifuge retention capacity (CRC) should not exceed 75g/g. If the centrifuge retention capacity (CRC) of the base polymer istoo high, it is not possible to build up sufficient absorption under apressure of 49.2 g/cm² (AUHL) in the subsequent thermal surfacepostcrosslinking.

Initiators c) used may be all compounds which generate free radicalsunder the polymerization conditions, for example thermal initiators,redox initiators or photoinitiators.

Suitable redox initiators are potassium peroxodisulfate or sodiumperoxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid,potassium peroxodisulfate or sodium peroxodisulfate/sodium bisulfite andhydrogen peroxide/sodium bisulfite. Preference is given to usingmixtures of thermal initiators and redox initiators, such as potassiumperoxodisulfate or sodium peroxodisulfate/hydrogen peroxide/ascorbicacid. The reducing component used is, however, preferably a mixture ofthe sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium saltof 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixturesare obtainable as Brüggolite® FF6 and Brüggolite® FF7 (BrüggemannChemicals; Heilbronn; Germany).

Suitable thermal initiators are especially azo initiators, such as2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride and2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride,2,2′-azobis(2-amidinopropane)dihydrochloride,4,4′-azobis(4-cyanopentanoic acid) and the sodium salts thereof,2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] and2,2′-azobis(imino-1 -pyrrolidino-2-ethylpropane)dihydrochloride.

Suitable photoinitiators are, for example,2-hydroxy-2-methylpropiophenone and1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one.

Ethylenically unsaturated monomers d) copolymerizable with theethylenically unsaturated monomers a) bearing acid groups are, forexample, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethylmethacrylate, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

The water-soluble polymers e) used may be polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, modified cellulose,such as methyl cellulose or hydroxyethyl cellulose, gelatin, polyglycolsor polyacrylic acids, preferably starch, starch derivatives and modifiedcellulose.

Optionally, one or more chelating agents may be added to the monomersolution or starting materials thereof to mask metal ions, for exampleiron, for the purpose of stabilization. Suitable chelating agents are,for example, alkali metal citrates, citric acid, alkali metal tartrates,pentasodium triphosphate, ethylenediamine tetraacetate, nitrilotriaceticacid, and also all chelating agents known by the Trilon® name, forexample Trilon® C (pentasodium diethylenetriaminepentaacetate), Trilon®D (trisodium (hydroxyethyl)ethylenediaminetriacetate), and Trilon® M(methylglycinediacetic acid).

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. The monomer solution can therefore be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingan inert gas through, preferably nitrogen or carbon dioxide. The oxygencontent of the monomer solution is preferably lowered before thepolymerization to less than 1 ppm by weight, more preferably to lessthan 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

If the polymerization is conducted under adequate reflux, theinertization can be dispensed with. In this case, the dissolved oxygenis removed from the polymerization reactor together with the evaporatingsolvent.

For polymerization, the monomer solution is suspended or emulsified in ahydrophobic solvent.

Usable hydrophobic solvents are all the solvents known to the personskilled in the art for use in suspension polymerization. Preference isgiven to using aliphatic hydrocarbons, such as n-hexane, n-heptane,n-octane, n-nonane, n-decane, cyclohexane or mixtures thereof.Hydrophobic solvents have a solubility in water at 23° C. of less than 5g/100 g, preferably less than 1 g/100 g, more preferably less than 0.5g/100 g.

The hydrophobic solvent boils within the range from preferably 50 to150° C., more preferably 60 to 120° C., most preferably 70 to 90° C.

The ratio between hydrophobic solvent and monomer solution is 0.5 to 3,preferably 0.7 to 2.5 and very preferably from 0.8 to 2.2.

The mean diameter of the monomer solution droplets in the suspension, ispreferably at least 100 μm, more preferably from 100 to 1000 μm, morepreferably from 150 to 850 μm, most preferably from 300 to 600 μm, thedroplet diameter being determinable by light scattering and signifyingthe volume-average mean diameter.

The diameter of the monomer solution droplets can be adjusted via thestirrer energy introduced and through suitable dispersing aids.

For dispersion of the aqueous monomer solution in the hydrophobicsolvent or for dispersion of the water-absorbing polymer particles whichform, dispersing aids are added. These dispersing aids may be anionic,cationic, nonionic or amphoteric surfactants, or natural, semisyntheticor synthetic polymers.

Anionic surfactants are, for example, sodium polyoxyethylene dodecylether sulfate and sodium dodecyl ether sulfate. A cationic surfactantis, for example, trimethylstearylammonium chloride. An amphotericsurfactant is, for example, carboxymethyldimethylcetylammonium. Nonionicsurfactants are, for example, sucrose fatty acid esters, such as sucrosemonostearate and sucrose dilaurate, sorbitan esters such as sorbitanmonostearate, polyoxyalkylene compounds based on sorbitan esters, suchas polyoxyethylenesorbitan monostearate.

The dispersing aid is typically dissolved or dispersed in thehydrophobic solvent. The dispersing aid is used in amounts between 0.01and 10% by weight, preferably between 0.2 and 5% by weight, morepreferably between 0.5 and 2% by weight, based on the monomer solution.The diameter of the monomer solution droplets can be adjusted via thetype and amount of dispersing aid.

Advantageously, several stirred reactors are connected in series for thepolymerization. Through postreaction in further stirred reactors, themonomer conversion can be increased and backmixing can be reduced. Inthis context, it is additionally advantageous when the first stirredreactor is not too large. With increasing size of the stirred reactor,there is inevitably broadening of the size distribution of the dispersedmonomer solution droplets. A relatively small first reactor thereforeenables the production of water-absorbing polymer particles with aparticularly narrow particle size distribution.

The reaction is preferably conducted under reduced pressure, for exampleat a pressure of 800 mbar. The pressure can be used to set the boilingpoint of the reaction mixture to the desired reaction temperature.

The resultant water-absorbing polymer particles are thermally surfacepostcrosslinked. The thermal surface postcrosslinking can be conductedin the polymer dispersion or with the water-absorbing polymer particleswhich have been removed from the polymer dispersion and dried.

In a preferred embodiment of the present invention, the water-absorbingpolymer particles are dewatered azeotropically in the polymer dispersionand removed from the polymer dispersion, and the water-absorbing polymerparticles removed are dried to remove the adhering residual hydrophobicsolvent and thermally surface postcrosslinked.

Suitable surface postcrosslinkers are compounds which comprise groupswhich can form covalent bonds with at least two carboxylate groups ofthe polymer particles. Suitable compounds are, for example,polyfunctional amines, polyfunctional amido amines, polyfunctionalepoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1,DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, asdescribed in DE 102 04 938 A1 and U.S. Pat. No. 6,239,230.

Additionally described as suitable surface postcrosslinkers are alkylenecarbonates in DE 40 20 780 C1, 2-oxazolidinone and derivatives thereof,such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazineand derivatives thereof in DE 198 54 573 A1, N-acyl-2-oxazolidinones inDE 198 54 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amidoacetals in DE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327A2 and morpholine-2,3-dione and derivatives thereof in WO 2003/031482A1.

In addition, it is also possible to use surface postcrosslinkers whichcomprise additional polymerizable ethylenically unsaturated groups, asdescribed in DE 37 13 601 A1.

In addition, it is possible to use any desired mixtures of the suitablesurface postcrosslinkers.

Preferred surface postcrosslinkers are alkylene carbonates,2-oxazolidinones, bis- and poly-2-oxazolidinones,2-oxotetrahydro-1,3-oxazines, N-acyl-2-oxazolidinones, cyclic ureas,bicyclic amido acetals, oxetanes and morpholine-2,3-diones.

Particularly preferred surface postcrosslinkers are ethylene carbonate(1,3-dioxolan-2-one), trimethylene carbonate (1,3-dioxan-2-one),3-methyl-3-oxethanemethanol, 2-hydroxyethyl-2-oxazolidinone,2-oxazolidinone and methyl-2-oxazolidinone.

Very particular preference is given to ethylene carbonate.

The amount of surface postcrosslinker is preferably 0.1 to 10% byweight, more preferably 0.5 to 7.5% by weight and most preferably 1 to5% by weight, based in each case on the polymer particles.

The surface postcrosslinkers are typically used in the form of anaqueous solution. The amount of the solvent is preferably 0.001 to 8% byweight, more preferably 2 to 7% by weight, even more preferably 3 to 6%by weight and especially 4 to 5% by weight, based in each case on thepolymer particles. The penetration depth of the surface postcrosslinkerinto the polymer particles can be adjusted via the content of nonaqueoussolvent and total amount of solvent.

When exclusively water is used as the solvent, a surfactant isadvantageously added. This improves the wetting characteristics andreduces the tendency to form lumps. However, preference is given tousing solvent mixtures, for example isopropanol/water,1,3-propanediol/water and propylene glycol/water, where the mixing ratioin terms of mass is preferably from 10:90 to 60:40.

In a preferred embodiment of the present invention, cations, especiallypolyvalent cations, are applied to the particle surface in addition tothe surface postcrosslinkers before, during or after the thermal surfacepostcrosslinking.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium, iron and strontium, trivalent cations such as thecations of aluminum, iron, chromium, rare earths and manganese,tetravalent cations such as the cations of titanium and zirconium.Possible counterions are hydroxide, chloride, bromide, sulfate,hydrogensulfate, carbonate, hydrogencarbonate, nitrate, phosphate,hydrogenphosphate, dihydrogenphosphate and carboxylate, such as acetate,citrate and lactate. Salts with different counterions are also possible,for example basic aluminum salts such as aluminum monoacetate oraluminum monolactate. Aluminum sulfate, aluminum monoacetate andaluminum lactate are preferred. Apart from metal salts, it is alsopossible to use polyamines as polyvalent cations.

The amount of polyvalent cation used is, for example, 0.001 to 1.5% byweight, preferably 0.005 to 1% by weight and more preferably 0.02 to0.8% by weight, based in each case on the polymer particles.

In a further preferred embodiment of the present invention,hydrophilizing agents are additionally applied before, during or afterthe thermal surface postcrosslinking, for example sugar alcohols such assorbitol, mannitol and xylitol, water-soluble polymers or copolymerssuch as cellulose, polyethylene glycols, polyvinyl alcohols,polyvinylpyrrolidones and polyacrylamides.

The surface postcrosslinking is typically performed in such a way that asolution of the surface postcrosslinker is sprayed onto the driedpolymer particles. After the spray application, the surfacepostcrosslinker-coated polymer particles are thermally surfacepostcrosslinked.

The spray application of a solution of the surface postcrosslinker ispreferably performed in mixers with moving mixing tools, such as screwmixers, disk mixers and paddle mixers. Particular preference is given tohorizontal mixers such as paddle mixers, very particular preference tovertical mixers. The distinction between horizontal mixers and verticalmixers is made by the position of the mixing shaft, i.e. horizontalmixers have a horizontally mounted mixing shaft and vertical mixers avertically mounted mixing shaft. Suitable mixers are, for example,horizontal Pflugschar® plowshare mixers (Gebr. Lödige Maschinenbau GmbH;Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV;Doetinchem; the Netherlands), Processall Mixmill mixers (ProcessallIncorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV;Doetinchem; the Netherlands). However, it is also possible to spray onthe surface postcrosslinker solution in a fluidized bed.

The thermal surface postcrosslinking is preferably performed in contactdriers, more preferably shovel driers, most preferably disk driers.Suitable driers are, for example, Hosokawa Bepex® Horizontal PaddleDryers (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® DiscDryers (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® driers(Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryers(NARA Machinery Europe; Frechen; Germany). Moreover, fluidized beddriers may also be used.

The thermal surface postcrosslinking can be effected in the mixeritself, by heating the jacket or blowing in warm air. Equally suitableis a downstream drier, for example a shelf drier, a rotary tube oven ora heatable screw. It is particularly advantageous to effect mixing andthermal surface postcrosslinking in a fluidized bed drier.

It may be advantageous to conduct the thermal surface postcrosslinkingunder reduced pressure or to conduct it with use of drying gases, forexample dried air and nitrogen, in order to ensure the very substantialremoval of the solvents.

Subsequently, the surface postcrosslinked polymer particles can beclassified, with removal of excessively small and/or excessively largepolymer particles and recycling thereof into the process.

The surface postcrosslinking can also be conducted in the polymerdispersion. For this purpose, the solution of the surfacepostcrosslinker is added to the polymer dispersion. In this context, itmay be advantageous to conduct the thermal surface postcrosslinkingunder elevated pressure, for example with use of hydrophobic organicsolvents having a boiling point at 1013 mbar below the desiredtemperature for the thermal surface postcrosslinking. After the thermalsurface postcrosslinking in the polymer dispersion, the water-absorbingpolymer particles are dewatered azeotropically in the polymer dispersionand removed from the polymer dispersion, and the water-absorbing polymerparticles removed are dried to remove the adhering residual hydrophobicsolvent.

Preferred surface postcrosslinking temperatures are in the range of 100to 190° C., preferably in the range of 105 to 180° C., more preferablyin the range of 110 to 175° C., most preferably in the range of 120 to170° C. The preferred residence time at this temperature is preferablyat least 10 minutes, more preferably at least 20 minutes, mostpreferably at least 30 minutes, and typically at most 90 minutes.

In a preferred embodiment of the present invention, the water-absorbingpolymer particles are cooled after the thermal surface postcrosslinkingin a contact drier. The cooling is preferably performed in contactcoolers, more preferably paddle coolers and most preferably diskcoolers. Suitable coolers are, for example, Hosokawa Bepex® HorizontalPaddle Coolers (Hosokawa Micron GmbH; Leingarten; Germany), HosokawaBepex® Disc Coolers (Hosokawa Micron GmbH; Leingarten; Germany),Holo-Flite® coolers (Metso Minerals Industries Inc.; Danville; USA) andNara Paddle Coolers (NARA Machinery Europe; Frechen; Germany). Moreover,fluidized bed coolers may also be used.

In the cooler, the water-absorbing polymer particles are cooled to 20 to150° C., preferably 30 to 120° C., more preferably 40 to 100° C. andmost preferably 50 to 80° C.

To further improve the properties, the polymer particles thermallysurface postcrosslinked in a contact drier can be coated orremoisturized.

The remoisturizing is preferably performed at 30 to 80° C., morepreferably at 35 to 70° C., most preferably at 40 to 60° C. Atexcessively low temperatures, the water-absorbing polymer particles tendto form lumps, and, at higher temperatures, water already evaporates toa noticeable degree. The amount of water used for remoisturizing ispreferably from 1 to 10% by weight, more preferably from 2 to 8% byweight and most preferably from 3 to 5% by weight. The remoisturizingincreases the mechanical stability of the polymer particles and reducestheir tendency to static charging.

Suitable coatings for improving the free swell rate and the permeability(SFC) are, for example, inorganic inert substances, such aswater-insoluble metal salts, organic polymers, cationic polymers and di-or polyvalent metal cations. Suitable coatings for dust binding are, forexample, polyols. Suitable coatings for counteracting the undesiredcaking tendency of the polymer particles are, for example, fumed silica,such as Aerosil® 200, and surfactants, such as Span® 20 and Plantacare818 UP and surfactant mixtures.

The present invention further provides the water-absorbing polymerparticles obtainable by the process according to the invention.

The water-absorbing polymer particles obtainable by the processaccording to the invention have a centrifuge retention capacity (CRC) ofat least 37 g/g, an absorption under a pressure of 21.0 g/cm² of atleast 30 g/g, an absorption under a pressure of 49.2 g/cm² (AUHL) of atleast 14 g/g, a sum total of centrifuge retention capacity andabsorption under a pressure of 21.0 g/cm² (CRC+AUL) of at least 69 g/g,a sum total of centrifuge retention capacity and absorption under apressure of 49.2 g/cm² (CRC+AUHL) of at least 54 g/g, and less than 20%by weight of extractables.

The inventive water-absorbing polymer particles have a centrifugeretention capacity (CRC) of preferably at least 38 g/g, more preferablyat least 40 g/g and most preferably at least 41 g/g. The centrifugeretention capacity (CRC) of the water-absorbing polymer particles istypically less than 75 g/g.

The inventive water-absorbing polymer particles have an absorption undera pressure of 21.0 g/cm² (AUL) of preferably at least 32 g/g, morepreferably at least 33 g/g and most preferably at least 34 g/g. Theabsorption under a pressure of 21.0 g/cm² (AUL) of the water-absorbingpolymer particles is typically less than 50 g/g.

The inventive water-absorbing polymer particles have an absorption undera pressure of 49.2 g/cm² (AUHL) of preferably at least 16 g/g, morepreferably at least 18 g/g and most preferably at least 20 g/g. Theabsorption under a pressure of 49.2 g/cm² (AUHL) of the water-absorbingpolymer particles is typically less than 35 g/g.

The sum total of centrifuge retention capacity (CRC) and absorptionunder a pressure of 21.0 g/cm² (AUL) of the inventive water-absorbingpolymer particles is preferably at least 71 g/g, more preferably atleast 73 g/g and most preferably at least 74 g/g.

The sum total of centrifuge retention capacity (CRC) and absorptionunder a pressure of 49.2 g/cm² (AUHL) of the inventive water-absorbingpolymer particles is preferably at least 56 g/g, more preferably atleast 58 g/g, most preferably at least 59 g/g.

The inventive water-absorbing polymer particles comprise preferably lessthan 17% by weight, more preferably less than 15% by weight and mostpreferably less than 14% by weight of extractables.

The inventive water-absorbing polymer particles have a proportion ofparticles having a particle size of 300 to 600 μm of preferably at least30% by weight, more preferably at least 40% by weight and mostpreferably at least 50% by weight.

The present invention further provides hygiene articles comprising

-   -   (A) an upper liquid-impermeable layer,    -   (B) a lower liquid-permeable layer,    -   (C) a liquid-absorbing storage layer between layer (A) and layer        (B), comprising from 0 to 30% by weight of a fibrous material        and from 70 to 100% by weight of water-absorbing polymer        particles obtainable by the process according to the invention,    -   (D) optionally an acquisition and distribution layer between        layer (A) and layer (C), comprising from 80 to 100% by weight of        a fibrous material and from 0 to 20% by weight of        water-absorbing polymer particles obtainable by the process        according to the invention,    -   (E) optionally a fabric layer directly above and/or beneath        layer (C) and    -   (F) further optional components.

The proportion of water-absorbing polymer particles obtainable by theprocess according to the invention in the liquid-absorbing storage layer(C) is preferably at least 75% by weight, more preferably at least 80%by weight, most preferably at least 90% by weight.

The mean sphericity of the water-absorbing polymer particles obtainableby the process according to the invention in the liquid-absorbingstorage layer (C) is 0.84, preferably at least 0.86, more preferably atleast 0.88, most preferably at least 0.90.

The water-absorbing polymer particles produced by customary solutionpolymerization (gel polymerization) are ground and classified afterdrying, to obtain irregular polymer particles. The mean sphericity ofthese water-absorbing polymer particles is between about 0.72 and about0.78.

The water-absorbing polymer particles are tested by means of the testmethods described below.

Methods:

The measurements should, unless stated otherwise, be conducted at anambient temperature of 23±2° C. and a relative air humidity of 50±10%.The water-absorbing polymers are mixed thoroughly before themeasurement.

Residual Monomers

The residual monomer content of the water-absorbing polymer particles isdetermined by EDANA recommended test method WSP No. 210.2-05 “ResidualMonomers”.

Sieve Analysis

The sieve analysis is conducted according to EDANA recommended testmethod WSP 220.3 (11), using sieves with the following mesh sizes: 100,200, 300, 400, 500, 600, 710, 800, 900 and 1000 μm.

The percentage of each fraction w is calculated as follows:

w=(m _(ni) −m _(si)×100/m ₁

where

m_(ni) is the polymer particle mass in g retained by each sieve

m_(si) is the mass of the empty sieve in g

m₁ is the total mass of polymer particles weighed in in g

Moisture Content

The moisture content of the water-absorbing polymer particles isdetermined by EDANA recommended test method No. WSP 230.3 (11) “MassLoss Upon Heating”.

Centrifuge Retention Capacity

The centrifuge retention capacity (CRC) is determined by EDANArecommended test method No. WSP 241.3 (11) “Fluid Retention Capacity inSaline, After Centrifugation”.

Absorption Under a Pressure of 0.0 g/cm²

The absorption under a pressure of 0.0 g/cm² (AUNL) is determinedanalogously to EDANA recommended test method No. WSP 242.3 (11)“Gravimetric Determination of Absorption Under Pressure”, except that apressure of 0.0 g/cm² (AUL 0.0 psi) is established instead of a pressureof 21.0 g/cm² (AUL 0.3 psi).

Absorption Under a Pressure of 21.0 g/cm²

The absorption under a pressure of 21.0 g/cm² (AUL) of thewater-absorbing polymer particles is determined by EDANA recommendedtest method No. WSP 242.3 (11) “Gravimetric Determination of AbsorptionUnder Pressure”.

Absorption Under a Pressure of 49.2 g/cm²

The absorption under a pressure of 49.2 g/cm² (AUHL) is determinedanalogously to EDANA recommended test method No. WSP 242.3 (11)“Gravimetric Determination of Absorption Under Pressure”, except that apressure of 49.2 g/cm² (AUL 0.7 psi) is established instead of apressure of 21.0 g/cm² (AUL0.3psi).

Bulk Density

The bulk density is determined by EDANA recommended test method No. WSP250.3 (11) “Gravimetric Determination of Density”.

Extractables

The content of extractables of the water-absorbing polymer particles isdetermined by EDANA recommended test method No. WSP 270.3 (11)“Extractable”. The extraction time is 16 hours.

Free Swell Rate

To determine the free swell rate (FSR), 1.00 g (=W1) of thewater-absorbing polymer particles is weighed into a 25 ml beaker anddistributed homogeneously over its base. Then 20 ml of a 0.9% by weightsodium chloride solution are metered into a second beaker by means of adispenser and the contents of this beaker are added rapidly to the firstand a stopwatch is started. As soon as the last drop of salt solutionhas been absorbed, which is recognized by the disappearance of thereflection on the liquid surface, the stopwatch is stopped. The exactamount of liquid which has been poured out of the second beaker andabsorbed by the polymer in the first beaker is determined accurately byreweighing the second beaker (=W2). The time interval required for theabsorption, which has been measured with the stopwatch, is designated ast. The disappearance of the last liquid droplet on the surface isdetermined as the time t.

The free swell rate (FSR) is calculated therefrom as follows:

FSR[g/g s]=W2/(W1×t)

If the moisture content of the water-absorbing polymer particles,however, is more than 3% by weight, the weight W1 should be corrected totake account of this moisture content.

Vortex Test

50.0 ml ±1.0 ml of a 0.9% by weight aqueous sodium chloride solution areintroduced into a 100 ml beaker which comprises a magnetic stirrer barof size 30 mm×6 mm. A magnetic stirrer is used to stir the sodiumchloride solution at 600 rpm. Then 2.000 g ±0.010 g of water-absorbingpolymer particles are added as rapidly as possible, and the time takenfor the stirrer vortex to disappear as a result of the absorption of thesodium chloride solution by the water-absorbing polymer particles ismeasured. When measuring this time, the entire contents of the beakermay still be rotating as a homogeneous gel mass, but the surface of thegelated sodium chloride solution must no longer exhibit any individualturbulences. The time taken is reported as the vortex.

Residual Cyclohexane The proportion of residual solvent (cyclohexane) isdetermined by means of headspace GC-MS.

Mean Sphericity (mSPHT)

The mean sphericity (mSPHT) is determined with the PartAn® 3001 Lparticle analyzer (Microtrac Europe GmbH; DE).

The sample to be analyzed is introduced into a funnel. Thecomputer-controlled measurement system starts the metering device andensures a continuous, concentration-regulated particle flow. Theparticles fall individually through the measurement shaft and generatehigh-contrast shadow images between light source and high-resolutioncamera. The light source is actuated by the camera and, because of veryshort exposure times, produces faultless image information for themultiple evaluation of each individual particle in real time.

In a 3D process, each particle is analyzed repeatedly and the processthus gives the absolute results for length, width, thickness, area andcircumference. The number of pixels covered by the particle is used tocalculate the size and shape. This also results in the comparativelyprecise determination of the mean sphericity (mSPHT) and the meanparticle diameter D₅₀.

EXAMPLES

Production of the Base Polymer:

Example 1

A 2L flange vessel equipped with impeller stirrer and reflux condenserwas initially charged with 896.00 g of cyclohexane and 6.00 g of ethylcellulose, and heated to internal temperature 75° C. with stirring andintroduction of nitrogen. The monomer solution, prepared from 150.00 g(2.082 mol) of acrylic acid, 129.00 g (1.613 mol) of 50% by weightaqueous sodium hydroxide solution, 136.80 g of water, 0.113 g (0.73mmol) of N,N′-methylenebisacrylamide (MBA) and 0.500 g (1.85 mmol) ofpotassium persulfate, was then introduced into a feed vessel and purgedwith air. Immediately prior to the dropwise addition of the monomersolution over a period of 1 h, the solution was inertized byintroduction of nitrogen. The stirrer speed was 300 rpm. Over the entireperiod over which the monomers were metered in, the reflux conditionswere maintained. The end of feeding was followed by the 60-minutefurther reaction period. Subsequently, the reflux condenser wasexchanged for a water separator and water was separated out.

The suspension present was cooled to 60° C. and the resultant polymerparticles were filtered off with suction using a Büchner funnel with apaper filter. The further drying was effected at 45° C. in an aircirculation drying cabinet and optionally in a vacuum drying cabinet at800 mbar down to a residual moisture content of less than 5% by weight.

The properties of the resultant polymer particles are summarized intables 2 and 3.

Examples 2 to 6

The base polymer was produced analogously to example 1 with the amountsstated in table 1.

The properties of the resultant polymer particles are summarized intables 2 and 3.

Example 7

The base polymer was produced analogously to example 4, with combinationof 30 batches.

The properties of the resultant polymer particles are summarized intables 2 and 3.

Example 8

A 2L flange vessel equipped with impeller stirrer and reflux condenserwas initially charged with 896.00 g of cyclohexane and 6.00 g of ethylcellulose, and heated to internal temperature 75° C. with stirring andintroduction of nitrogen. The monomer solution, prepared from 150.00 g(2.082 mol) of acrylic acid, 129.00 g (1.613 mol) of 50% by weightaqueous sodium hydroxide solution, 136.80 g of water, 0.113 g (0.73mmol) of N,N′-methylenebisacrylamide (MBA), 0.250 g (0.925 mmol) ofpotassium persulfate and 2.250 g of an 11.1% aqueous solution of2,2′-azobis(imino-1-pyrrolidino-2-ethylpropane)dihydrochloride (0.711mmol), was then introduced into a feed vessel and purged with air.Immediately prior to the dropwise addition of the monomer solution overa period of 1 h, the solution was inertized by introduction of nitrogen.The stirrer speed was 300 rpm. Over the entire period over which themonomers were metered in, the reflux conditions were maintained. The endof feeding was followed by the 60-minute further reaction period.Subsequently, the reflux condenser was exchanged for a water separatorand water was separated out.

The suspension present was cooled to 60° C. and the resultant polymerparticles were filtered off with suction using a Büchner funnel with apaper filter. The further drying was effected at 45° C. in an aircirculation drying cabinet and optionally in a vacuum drying cabinet at800 mbar down to a residual moisture content of less than 5% by weight.

The properties of the resultant polymer particles are summarized intables 2 and 3.

Example 9

The base polymer was produced analogously to example 1 using 0.075 g(0.194 mmol) of the triacrylate of 3-tuply ethoxylated glycerol(Gly-(EO-AA)₃) rather than 0.113 g (0.73 mmol) ofN,N′-methylenebisacrylamide (MBA) as internal crosslinker.

The properties of the resultant polymer particles are summarized intables 2 and 3.

Example 10

The base polymer was produced analogously to example 9, except using118.00 g (1.475 mol) of 50% by weight aqueous sodium hydroxide solutionrather than 129.00 g (1.613 mol) of 50% by weight aqueous sodiumhydroxide solution.

The properties of the resultant polymer particles are summarized intables 2 and 3.

TABLE 1 Amounts of crosslinker b) used Ex. Crosslinker b) g mmol ppmboaa mmol % boaa 1 MBA 0.1125 0.730 750 35 2 MBA 0.0750 0.486 500 23 3MBA 0.0563 0.365 375 18 4 MBA 0.0375 0.243 250 12 5 MBA 0.0188 0.122 1256 6 MBA 0.0000 0.000 0 0 8 MBA 0.0375 0.243 250 12 9 Gly-(EO-AA)₃ 0.07500.194 500 9 10 Gly-(EO-AA)₃ 0.0750 0.194 500 9 boaa: based on(unneutralized) acrylic acid MBA: methylenebisacrylamide Gly-(EO-AA)₃triacrylate of 3-tuply ethoxylated glycerol

TABLE 2 Properties of the water-absorbing polymer particles (basepolymer) CRC AUNL AUL AUHL Bulk density Residual monomers Residualcyclohexane Ex. g/g g/g g/g g/g g/100 ml Moisture content % Extractables% ppm ppm 1 33.6 41.5 24.5 16.5 94 3.6 8 12 380 2 31.1 37.0 22.1 13.6 9810.1 7 0 200 3 39.9 46.4 22.2 9.8 102 2.7 16 26 260 4 43.6 47.4 15.6 9.2102 2.8 13 24 220 5 50.8 53.4 10.2 7.6 102 3.7 20 15 200 6 61.7 53.4 7.66.4 102 2.8 31 23 170 7 41.6 45.6 20.3 8.5 99 2.8 13 14 210 8 49.6 48.48.7 7.4 101 3.4 17 39 200 9 45.3 50.8 15.5 7.2 100 1.9 14 21 210 10 45.249.9 12.6 7.2 101 3.0 16 24 190

TABLE 3 Sieve analysis (base polymer) Sieve analysis in % by weight800-900 >1000 Ex. <100 μm 100-200 μm 200-300 μm 300-400 μm 400-500 μm500-600 μm 600-710 μm 710-800 μm μm 900-1000 μm μm 1 1 7 28 37 16 4 3 11 0 2 2 1 9 26 37 19 5 2 0 0 0 0 3 1 7 27 36 18 5 3 1 1 1 1 4 1 6 24 3719 5 4 2 1 1 0 5 0 5 21 33 19 7 6 3 2 1 2 6 1 10 33 39 13 2 1 0 0 0 0 70 3 17 31 23 10 8 3 2 1 2 8 0 3 13 24 19 9 11 6 6 4 5 9 0 8 32 37 15 4 21 0 0 0 10 0 4 20 29 17 8 10 6 4 1 1

Thermal Surface Postcrosslinking:

Examples 1-1 and 1-2

20 g of base polymer from example 1 were introduced into a Waring®32BL80 (8011) blender. Subsequently, the Waring® blender was switched onat level 1. Immediately thereafter, 1.5 g of an aqueous solutionconsisting of 0.5 g of ethylene carbonate and 1.0 g of water, accordingto table 4, were introduced into a pipette and metered into the blenderwithin 2 sec. After 3 sec, the Waring® blender was switched off and theresultant polymer particles were distributed homogeneously in a glassdish having a diameter of 20 cm. For thermal surface postcrosslinking,the glass dish filled with the polymer particles was heated in an aircirculation drying cabinet at 160° C. for 60 or 75 min. The polymerparticles were transferred to a cold glass dish. Finally, the coarserparticles were removed with a sieve having a mesh size of 850 μm.

The properties of the polymer particles are summarized in table 5.

Examples 2-1 and 2-2

The thermal surface postcrosslinking was effected analogously to example1-1 and 1-2, except using the base polymer from example 2. The heattreatment time was 60 or 75 min. The conditions are summarized in table4.

The properties of the polymer particles are summarized in table 5.

Examples 3-1 and 3-2

The thermal surface postcrosslinking was effected analogously to example1-1 and 1-2, except using the base polymer from example 3. The heattreatment time was 75 or 90 min. The conditions are summarized in table4.

The properties of the polymer particles are summarized in table 5.

Examples 4-1 and 4-2

The thermal surface postcrosslinking was effected analogously to example1-1 and 1-2, except using the base polymer from example 4. The heattreatment time was 60 or 75 min. The conditions are summarized in table4.

The properties of the polymer particles are summarized in table 5.

Example 4-3

The thermal surface postcrosslinking was effected analogously to example1-1, except using the base polymer from example 4 and additionally usingaluminum trilactate. The heat treatment time was 90 min. The conditionsare summarized in table 4.

The properties of the polymer particles are summarized in table 5.

Examples 4-4 and 4-5

The thermal surface postcrosslinking was effected analogously to example1-1 and 1-2, except using the base polymer from example 4 and usingN,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamine (Primid® XL 552) assurface postcrosslinker. The heat treatment time was 60 or 75 min. Theconditions are summarized in table 4.

The properties of the polymer particles are summarized in table 5.

Examples 5-1 and 5-2

The thermal surface postcrosslinking was effected analogously to example1-1 and 1-2, except using the base polymer from example 2. The heattreatment time was 75 or 90 min. The conditions are summarized in table4.

The properties of the polymer particles are summarized in table 5.

Examples 6-1 and 6-2

The thermal surface postcrosslinking was effected analogously to example1-1 and 1-2, except using the base polymer from example 6. The heattreatment time was 60 or 75 min. The conditions are summarized in table4.

The properties of the polymer particles are summarized in table 5.

Examples 7-1 to 7-2

1.5 kg of water-absorbing polymer particles from example 7 wereintroduced at 23° C. into a Pflugschar® M5R paddle drier (Gebr. LödigeMaschinenbau GmbH, Paderborn, Germany), and a speed of 200 rpm was seton the Pflugschar® paddle drier. A solution consisting of 37.5 g ofethylene carbonate and 75.0 g of water was sprayed onto the product fromabove by means of a Büchi two-phase nozzle with 1 bar nitrogen withinabout 2 min, and then stirring of the product mixture continued forabout 5 min.

Subsequently, the product was transferred to a further Pflugschar®paddle drier. The Pflugschar® paddle drier had been preheated to a walltemperature of 190° C. Subsequently, the Pflugschar® paddle drier wasset to a speed of 200 rpm. The temperature fell significantly as aresult of the introduction of the product. The stirrer was started. Onattainment of a product temperature of 143° C., the thermostat for theoil heating was turned down from 250° C. to 190° C. During theexperiment, the heating was regulated such that a constant producttemperature of 160° C. was established after about 20 min. The cooledproduct was sieved down to smaller than 850 μm on an AS400 sieve shaker(Retsch GmbH, Haan, Germany).

The properties of the polymer particles are summarized in tables 6 and7.

Examples 7-3 and 7-4

The thermal surface postcrosslinking was effected analogously to example7-1 and 7-2, except at lower temperature.

The Pflugschar® paddle drier had been preheated to a wall temperature of110° C. The temperature fell significantly as a result of theintroduction of the product. On attainment of a product temperature of83° C., the thermostat for the oil heating was turned down from 250° C.to 110° C. During the experiment, the heating was regulated such that aconstant product temperature of 90° C. was established after about 20min.

The properties of the polymer particles are summarized in table 6.

Examples 7-5 and 7-6

The thermal surface postcrosslinking was effected analogously to example7-1 and 7-2, except at higher temperature.

The Pflugschar® paddle drier had been preheated to a wall temperature of220° C. The temperature fell significantly as a result of theintroduction of the product. On attainment of a product temperature of183° C., the thermostat for the oil heating was turned down from 250° C.to 230° C. During the experiment, the heating was regulated such that aconstant product temperature of 200° C. was established after about 20min.

The properties of the polymer particles are summarized in table 6.

Examples 8-1, 8-2, 9-1 and 10-1

The thermal surface postcrosslinking was effected analogously to example1-1, except using the base polymer from example 8, 9 or 10. Theconditions are summarized in table 4.

TABLE 4 Thermal surface postcrosslinking in Waring ® blender -conditions Ethylene Temperature Time carbonate Water Al lactate Primid ®XL 552 Ex. Crosslinker b) ° C. min % by wt. bop % by wt. bop % by wt.bop % by wt. bop  1 750 ppm MBA — — — — — —  1-1*) 160 60 2.5 5 — — 1-2*) 160 75 2.5 5 — —  2 500 ppm MBA — — — — — —  2-1*) 160 60 2.5 5 ——  2-2*) 160 75 2.5 5 — —  3 375 ppm MBA — — — — — —  3-1 160 75 2.5 5 ——  3-2 160 90 2.5 5 — —  4 250 ppm MBA — — — — — —  4-1 160 60 2.5 5 — — 4-2 160 75 2.5 5 — —  4-3 160 75 2.5 5 0.5 —  4-4 160 75 — 5 — 0.25 4-5 160 90 — 5 — 0.25  5 125 ppm MBA — — — — — —  5-1 160 75 2.5 5 — — 5-2 160 90 2.5 5 — —  6 0 ppm MBA — — — — — —  6-1 160 60 2.5 5 — — 6-2 160 75 2.5 5 — —  8 250 ppm MBA — — — — — —  8-1 160 60 2.5 5 — — 8-2 160 75 2.5 5 — —  9 500 ppm — — — — — —  9-1 Gly-(EO-AA)₃ 160 602.5 5 — — 10 500 ppm — — — — — — 10-1 Gly-(EO-AA)₃ 160 60 2.5 5 — —*)comparative example

TABLE 5 Thermal surface postcrosslinking in Waring ® blender -properties of the polymer particles CRC AUNL AUL AUHL Moisture contentExtractables Vortex FSR Bulk density CRC + AUL CRC + AUHL Ex. g/g g/gg/g g/g % by wt. % by wt. s g/g s g/100 ml g/g g/g  1 33.6 41.5 24.516.5 3.6 8 94 58.1 50.1  1-1*) 35.0 45.4 31.5 24.3 1.7 3 101 0.09 9666.5 59.3  1-2*) 33.7 46.7 32.9 25.0 1.5 3 112 0.10 97 66.6 58.7  2 31.137.0 22.1 13.6 10.1 7 98 53.2 44.7  2-1*) 35.8 47.0 32.2 25.0 1.3 4 1170.09 101 68.0 60.8  2-2*) 35.1 45.5 32.4 24.9 1.1 4 125 0.09 102 67.560.0  3 39.9 46.4 22.2 9.8 2.7 16 102 62.1 49.7  3-1 43.8 52.6 32.6 19.41.3 9 126 0.14 101 76.4 63.2  3-2 42.9 52.8 31.1 19.1 1.2 6 189 0.09 10274.0 62.0  4 45.3 49.4 18.9 7.3 3.1 13 102 64.2 52.6  4-1 45.3 47.0 29.013.2 1.2 5 107 0.11 100 74.3 58.5  4-2 45.2 45.5 30.2 19.0 1.1 4 1120.09 102 75.4 64.2  4-3 38.8 50.1 30.8 16.0 1.5 10.8 152 0.17 99 69.654.7  4-4 41.1 41.5 23.5 10.0 1.2 15.0 179 0.13 100 64.5 51.1  4-5 41.845.2 24.3 13.3 1.2 16.6 178 0.11 101 66.1 55.1  5 50.8 53.4 10.2 7.2 3.720 102 61.0 58.0  5-1 47.3 57.1 28.2 10.2 1.5 8 131 0.10 99 75.5 57.5 5-2 49.4 58.0 25.3 12.4 1.3 6 125 0.08 102 74.7 61.8  6 61.7 53.4 7.66.4 2.8 31 102 69.3 68.1  6-1 59.3 64.9 18.3 8.3 1.4 10 102 0.13 10277.6 67.6  6-2 54.8 65.7 18.1 9.5 1.3 11  91 0.14 103 76.5 64.3  8 49.648.4 8.7 7.4 3.4 17 166 — 101 58.3 57.0  8-1 44.3 52.1 32.6 20.7 1.2 9105 0.14 98 76.9 61.0  8-2 44.1 51.5 34.7 23.9 0.9 5 110 0.13 97 78.868.0  9 45.3 50.8 15.5 7.2 1.9 14 170 — 100 60.8 52.5  9-1 50.2 63.033.8 14.0 0.4 5 112 0.13 103 84.0 64.2 10 45.2 49.9 12.6 7.2 3.0 16 — —101 54.8 52.4 10-1 39.3 54.4 31.4 20.8 1.0 14 255 0.08 103 70.7 60.1*)comparative example

TABLE 6 Thermal surface postcrosslinking in a Pflugschar ® paddledrier - influence of temperature Moisture Bulk Temperature Time CRC AUNLAUL AUHL content Extractables FSR density CRC + AUL CRC + AUHL Ex. ° C.min g/g g/g g/g g/g % by wt. % by wt. Vortex s g/g s g/100 ml g/g g/g 7— 41.6 45.6 20.3 8.5 2.8 8.8 155 0.1 99 59.1 50.1 7-1 160 40 37.8 52.935.8 23.1 1.3 3.3 149 0.1 102 72.2 60.9 7-2 160 60 36.0 49.7 33.4 24.11.1 8.7 143 0.1 101 53.5 60.1 7-3*) 90 40 38.7 43.7 19.5 7.5 4.8 10.1165 0.1 94 58.2 46.2 7-4*) 90 60 39.2 44.2 19.1 7.4 4.3 10.3 172 0.1 9358.3 46.6 7-5*) 200 40 33.8 38.1 30.0 21.0 1.1 11.1 145 0.1 99 63.8 54.87-6*) 200 60 31.9 36.6 29.6 18.8 0.5 13.9 181 0.1 99 61.5 50.7*)comparative example

TABLE 7 Thermal surface postcrosslinking in a Pflugschar ® paddledrier - Analysis with a PartAn ® 3001 L particle analyzer Ex. Meansphericity (mSPHT) Mean particle diameter (D₅₀) 7 0.89 381 μm 7-1 0.90379 μm 7-2 0.90 374 μm

1. A process for producing water-absorbing polymer particles bypolymerizing a monomer solution comprising a) at least one ethylenicallyunsaturated monomer which bears an acid group and optionally at leastpartly neutralized, b) optionally one or more crosslinker, c) at leastone initiator, d) optionally one or more ethylenically unsaturatedmonomer copolymerizable with the monomer mentioned under a) and e)optionally one or more water-soluble polymer, the monomer solution beingsuspended in a hydrophobic organic solvent during the polymerization,and thermally surface postcrosslinking the resultant polymer particlesusing an organic surface postcrosslinker, wherein the amount ofcrosslinker b) is selected such that the polymer particles before thesurface postcrosslinking have a centrifuge retention capacity of atleast 37 g/g and the thermal surface postcrosslinking is conducted at100 to 190° C.
 2. The process according to claim 1, wherein the amountof crosslinker b) is selected such that the polymer particles before thesurface postcrosslinking have a centrifuge retention capacity of atleast 40 g/g.
 3. The process according to claim 1, wherein the thermalsurface postcrosslinking is conducted at 120 to 170° C.
 4. The processaccording to claim 1, wherein the organic surface postcrosslinker isselected from the group consisting of alkylene carbonates,2-oxazolidinones, bis- and poly-2-oxazolidinones,2-oxotetrahydro-1,3-oxazines, N-acyl-2-oxazolidinones, cyclic ureas,bicyclic amido acetals, oxetanes, and morpholine-2,3 -diones.
 5. Theprocess according to claim 1, wherein from 1 to 5% by weight of organicsurface postcrosslinker is used, based on the resultant polymerparticles.
 6. The process according to claims 1, wherein a mean dropletdiameter of the suspended monomer solution is from 200 to 500 μm.
 7. Theprocess according to any of claim 1, wherein a dispersing aid is used inthe polymerization.
 8. The process according to claim 1, wherein theresultant polymer particles are dewatered azeotropically after thepolymerization.
 9. The process according to claim 7, wherein theresultant polymer particles are filtered and dried after the azeotropicdewatering.
 10. The process according to claim 1, wherein the thermalsurface postcrosslinking is conducted in a mixer with moving mixingtools.
 11. Water-absorbing polymer particles obtained by a process ofclaim 1, having a centrifuge retention capacity of at least 37 g/g, anabsorption under a pressure of 21.0 g/cm² of at least 30 g/g, anabsorption under a pressure of 49.2 g/cm² of at least 14 g/g, a sumtotal of centrifuge retention capacity and absorption under a pressureof 21.0 g/cm² of at least 69 g/g, a sum total of centrifuge retentioncapacity and absorption under a pressure of 49.2 g/cm² of at least 54g/g, and less than 20% by weight of extractables.
 12. Water-absorbingpolymer particles according to claim 11, having a centrifuge retentioncapacity of at least 41 g/g.
 13. Water-absorbing polymer particlesaccording to claim 11, having an absorption under a pressure of 21.0g/cm² of at least 34 g/g.
 14. Water-absorbing polymer particlesaccording to claim 11, having an absorption under a pressure of 49.2g/cm² of at least 20 g/g.
 15. Water-absorbing polymer particlesaccording to claim 11, having a sum total of centrifuge retentioncapacity and absorption under a pressure of 21.0 g/cm² of at least 74g/g.
 16. Water-absorbing polymer particles according to claim 11, havinga sum total of centrifuge retention capacity and absorption under apressure of 49.2 g/cm² of at least 59 g/g.
 17. Water-absorbing polymerparticles according to claim 11, having less than 14% by weight ofextractables.
 18. Water-absorbing polymer particles according to claim11, having a bulk density of at least 1.0 g/cm³.
 19. Water-absorbingpolymer particles according to claim 11, wherein a proportion ofparticles having a particle size of 300 to 600 μm is at least 30% byweight.
 20. A hygiene article comprising (A) an upper liquid-impermeablelayer, (B) a lower liquid-permeable layer, (C) a liquid-absorbingstorage layer between layer (A) and layer (B), comprising from 0 to 30%by weight of a fibrous material and from 70 to 100% by weight ofwater-absorbing polymer particles, (D) optionally an acquisition anddistribution layer between layer (A) and layer (C), comprising from 80to 100% by weight of a fibrous material and from 0 to 20% by weight ofwater-absorbing polymer particles, (E) optionally a fabric layerdirectly above and/or beneath layer (C) and (F) further optionalcomponents wherein the water-absorbing polymer particles of (C) and (D)are according to claim
 11. 21. The hygiene article according to claim20, wherein the water-absorbing polymer particles have a mean sphericityof at least 0.84.